Foreign object damage (FOD) on aircraft turbine engine blades is a condition defined as any damage from small nicks and scratches to complete disablement or destruction of an engine caused by damaged parts of blades that break loose during operation. The blades rotate at very high speeds, and various levels of damage can be inflicted by foreign objects striking the blades, depending on the speed of the blades, the speed of the aircraft, and the size and hardness of the object.
FOD can be caused in a variety of ways including rocks or other hard objects being ingested by the engine during operation in the air or on the ground. Another source of FOD arises when components such as nuts, bolts, or tools are mistakenly left in or near the engine during maintenance. Additionally, leading and trailing edges of blades are subject to gradual erosion from abrasive sand and grit particles in the air that is ingested by the engine during operation. Another common source of engine FOD is in-flight icing that forms on the airframe or engine itself, breaks off and is ingested by the engine.
A typical engine has hundreds of blades which must be inspected for cracks, foreign object damage (FOD), and corrosion. Damaged and eroded blades degrade engine performance, as well as posing a potential cause of engine failure, and therefore must be replaced once the damage or erosion has exceeded a certain level. The flight crew is often not aware of FOD, however, in case of extensive damage, it can be indicated by vibration and changes in the engine's normal operating parameters. For example, damage to fan or compressor blades results in an increase in exhaust gas temperature (EGT), decrease in engine pressure ratio (EPR), and a change in the speed (RPM) ratio between the core engine and the fan section (N2/N1). Visual inspection for substantial damage to inlet fan blades and guide vanes is required to determine whether a blade can be repaired, or must be replaced.
A certain level of damage to a blade does not pose a serious threat to operational safety or performance. Guidelines regarding the extent of visible blade damage allowed are often provided to enable maintenance personnel to judge whether the blade must be repaired or replaced before further use. The amount of allowable damage can vary depending on the location of the damage along the length of the blade. The guidelines are typically expressed in terms of a percentage or amount of the leading and trailing edges of the chord (width) of the blade that may be missing before repair or replacement is required.
It is often difficult to visually gauge the amount of a blade that is missing without a measuring aid, such as a ruler. One known method of visual inspection includes marking damage limits at the tip and the bottom (root) of the blade. A straight line is drawn along the length of the blade between the marks at the tip and the root. The marks can be drawn on the leading edge and/or the trailing edge, depending whether both edges of the blade are damaged.
Once the allowable limits for damage to the blade are marked on the blade, the inspector can determine whether the blade must be replaced. When a damaged area on a blade is determined to be within a tolerable range, it is often desirable to blend the sharp edges of the damage to improve aerodynamic performance of the blade. The blending process also helps relieve stress points, thereby helping to avoid further cracking/damage to the blade during operation of the engine. Typically, an inspector will mark a small arc around the damaged area to indicate the area to be blended.
Thus, the inspector must measure along the length and the width of the blade to gauge each area of damage on both the leading and the trailing edges. Typically, a blade will have an aerodynamic contour that includes a cambered airfoil along the chord with a slight twist along the length of the blade. The contour can add to the difficulty of measuring the blade with a conventional straightedge ruler. Additionally, fan and compressor blades are typically tapered along their length from root to tip, further adding to the difficulty of determining whether the damage is within allowable limits. The problem is exacerbated on aircraft with inlet ducts, which do not allow easy access to inspect the blades.
A visual inspection of the engine fan blades is typically conducted on a routine basis. Considering the number of jet engines in service, millions of man-hours are required each year to complete the inspections. The blade inspection procedure outlined above is awkward, time-consuming, and prone to human error. It is therefore desirable to provide a device to facilitate more accurate and efficient visual fan and compressor blade inspection. It is also desirable to include a guide for marking the blend areas on the damaged portions of the blade with the inspection device.